Background for the present invention is provided herein in connection with a disk drive system. It should be noted, however, that the present invention is not intended to be limited to such systems.
A disk drive is a data storage device that stores digital data in tracks on the surface of a data storage disk. Data is read from or written to a track of the disk using a transducer that is held close to the track while the disk spins about its center at a substantially constant angular velocity. To properly locate the transducer near the desired track during a read or write operation, a closed-loop servo scheme is generally implemented that uses servo data read from the disk surface to align the transducer with the desired track.
The servo data includes servo patterns that typically comprise short servo bursts of a constant frequency signal, which are very precisely located and offset from either side of a data track's centerline. The bursts are written in a sector header area, and can be used to find the centerline of a track. Staying on center is required during both reading and writing. These servo-data areas allow a head to follow a track centerline around a disk, even when the track is out-of-round, as can occur with spindle wobble, disk slip and/or thermal expansion.
Servo bursts are conventionally written on a disk in the disk drive by a dedicated, external servo track writer (STW), which typically involves the use of large granite blocks to support the disk drive and to quiet outside vibration effects. However, servo track writers are expensive and require a clean room environment. As such, self-servo writing (SSW) methods for writing servo patterns with a disk drive's own transducers have been utilized.
Typically, in a SSW process, a temporary set of pre-existing reference servo information on a disk is used to control the transducer position while the final servo bursts are written to disk(s) in the disk drive. The SSW process involves a combination of three largely distinct sub-processes, including reading the temporary servo information to provide precise timing information, positioning a transducer at a sequence of radial positions using the variation in a read back signal amplitude as a sensitive position indicator, and writing the final servo burst patterns at the times and radial positions defined by the other two processes to form concentric circular tracks. An example SSW process is described in U.S. Pat. No. 5,907,447, by Yarmchuk, et al. Other SSW to processes are possible, such as servo propagation where the servo reader to writer offset is used to allow servoing on one set of servo bursts while writing another set of servo bursts.
In an ideal disk drive system, the tracks of the data disk are non-perturbed circles situated about the center of the disk. As such, each of these ideal tracks includes a track centerline that is located at a known constant radius from the disk center. In an actual system, however, it is difficult to write non-perturbed circular tracks to the data storage disk. That is, problems, such as vibration, bearing defects, etc. can result in tracks that are written differently from the ideal non-perturbed circular track shape. Positioning errors created by the perturbed nature of these tracks are known as written-in repetitive runout (SSW—RRO). The perturbed shape of these tracks complicates the transducer positioning function during read and write operations after the SSW process because the servo system needs to continuously reposition the transducer during track following to keep up with the constantly changing radius of the track centerline with respect to the center of the spinning disk. Furthermore, the perturbed shape of the these tracks can result in problems such as track squeeze and track misregistration errors during read and write operations.
In certain systems, as will be understood by those skilled in the art, after the servo patterns are written, an additional process is used to directly measure the SSW—RRO for each track of a disk so that compensation values are generated and written in servo fields on the disk. Thereafter, during read/write operations, that compensation information is used to position the transducer along an ideal track centerline. An example of such a process is described in U.S. Pat. No. 6,549,362 to Melrose et al. ('362 patent), which is incorporated herein by reference.
However, such a correction technique though effective, can be time consuming. First, the amount of SSW—RRO present on each track of a disk must be measured, and then a calculation is performed to determine correction factors to minimize the SSW—RRO in each track. Finally, the correction factors must be written to the disk in each servo field of each track. This process requires several revolutions to measure the SSW—RRO and then more revolutions to write the correction factors to the disk. In one example, such a process may require 12 or more revolutions to determine and write correction factors for each track.
There is, therefore, a need for a method and apparatus which improves embedded runout correction in a disk drive during the self-servo writing process and which also reduces the correction time required.